This invention relates to packaging machinery, and in particular to a system for processing a group of items and sealing them in a plastic film as the items follow a path of travel.
The invention is directed to horizontal form, fill and seal machinery, and particularly that type of machinery in which items to be packaged are enveloped in a plastic film. Other applications of the invention are possible, however.
The heart of a form, fill and seal machine is the sealing apparatus. No matter how quickly items to be packaged might be collected and grouped, it is imperative that they be fully sealed in a package that will not inadvertently open, which is as tightly sealed as desired, and which has all graphics properly displayed and aligned in the packaging material.
The first horizontal form, fill and seal machine was developed by Sam Campbell of Hudson Sharp Company, Green Bay, Wis. in the late 1940's. This machine consisted of a product infeed conveyor, a horizontal forming shoulder for enveloping items within a packaging material, a rotary back seam sealing device which acted as the prime mover for the film, and horizontally rotating sealing bars which both sealed and crush cut the wrapping material. This machine was designed for high speed overwrapping of candy bars and the like, and became known in the industry as the "Campbell Wrapper".
When the Campbell Wrapper was developed, wrapping materials prevalent at the time were heat sealing cellophane and heat seal paper using nitrocellulose coatings as the heat seal medium. It was well known at the time, and is still well known, that in order to obtain a good heat seal, three fundamental criteria must be present--time, temperature and pressure. Without these three criteria in the right proportions, efficient heat sealing cannot be obtained. As early as the 1930's, the DuPont Company, the inventors of heat sealable and moisture proof cellophane, evolved a testing method utilizing time, temperature and pressure in order to provide a standard against which the integrity of heat seals is measured.
With the introduction of polyethylene and other plastic films, the same criteria remain true at the present time. However, because heated plastics lose their structural integrity, cooling must also be a factor in the formula.
One of the most important speed inhibiting factors of the Campbell Wrapper was the very short sealing or dwell time utilized for sealing of the wrapping material. The single rotating horizontal sealing heads of the Campbell Wrapper have their surfaces curved in order to maintain sealing contact. This requires that the time that the sealing heads are in contact with the wrapping material is very short. As a result, the short dwell time has been compensated for by relatively high unit pressure and high temperatures.
Longer dwell times with lower temperatures was a goal constantly sought. Several attempts were made to overcome the short dwell time problem of the Campbell Wrapper, and in the late 1950's or early 60's, a system was developed utilizing multiple heat sealing jaws located on top and bottom chains, the pitch of the sealing jaws being equal to the finished length of the package. This added substantial dwell time, and in fact produced an improved Campbell Wrapper having very high speed. However, due to the configuration of this improvement, in order to change a finished package length, the entire top and bottom sealing systems had to be replaced with a different set of sealing jaws on different chains with the appropriate pitch to coincide with the new package length. While machines of this type were made, the difficulties of changing from one size to another were a severely inhibiting factor.
Another attempt to increase dwell time was to include two horizontal sealing heads in a machine, the first sealing head imparting a "preheat" and the second sealing head adding additional heat and severing the material. Although this configuration did increase speed somewhat, the additional dwell time achieved was marginal.
A further attempt to increase dwell time was to have an individual sealing die close, move in the direction of the film for a short distance, open and then return to the ready position for the next seal. Yet another system developed had rotating dies operating in a "D" motion with the flat side of the D being the dwell. In all cases where dwell time was extended on a single die machine, the mechanics of the systems have been speed inhibiting, and at the most give a dwell time of 50% of an individual package making time, i.e. at 60 per minute the maximum dwell time would be 500 milliseconds.
The first multi-die machine, with the ability to maintain the flexibility of being able to change over easily from one package to another, was designed in the late 1950's by the Cloud Corporation, Chicago, Ill., and was patented. The Cloud patent was subsequently purchased by The Hayssen Manufacturing Company, the assignee of the present application. Hayssen still manufactures equipment covered by the Cloud patent. The Cloud device consists of a series of hinged sealing dies which are slidably mounted on a drum, each die having a mechanism for allowing it to either slide on the drum or grip and travel with the drum. In practice a series of the dies are backed up with the foremost die being stationary in the 12 o'clock position. A release mechanism, operated by a signal from either the product itself in the case of an unprinted film or by a registration mark on the film when proper orientation of graphics is necessary, releases a die which grips the drum and rotates with the drum. The hinged top die closes and is held under pressure while the sealing and cutting takes place. At a speed of 60 packages per minute and a package length of approximately 8 inches, up to 5 seconds of dwell time is achieved compared to something less than 250 milliseconds on a standard Campbell Wrapper machine.
Another phenomenon in horizontal form, fill and seal packaging machines must also be considered. This is the pitch length of the product in the unclosed tube of packaging material verses the finished package length. This is due to the fact that as the packaging material is collapsed between succeeding products or groups of products in the tube, the distance between the products must be reduced in order to obtain a seal, or else the packaging material is stretched and quite probably weakened or damaged. In order to obtain a very tight package, the distance between items in the tube being sealed is one product height plus the width of the sealing bar. Thus, as the product height increases, the differential between the length of pitch between items in the tube and the finished package length becomes greater and greater.
This phenomenon is not a problem in a Campbell Wrapper, which has only a single die, since only one seal and cut is made at a time. Therefore, a package being sealed and cut can change velocity without restraint during the sealing and cutting operation. However, whenever multiple dies are in contact with the film, and where these multiple dies are operated at a constant velocity in a single drive, this phenomenon is accommodated by having the collapsing sealing head pull excess film from downstream. This results in a jerking motion of the film, and is an undesirable characteristic, particularly where several products are simultaneously being packaged within the tube. The incoming film must therefore operate at a variable velocity.
Cassoli U.S. Pat. No. 4,679,379 is directed to a horizontal form, fill and seal machine for packaging of large bundles of toilet tissue and household towels. This machine has single dies with product being intermittently fed through the sealing and wrapping area and intermittently discharged. However, the apparatus has a single die and intermittent operation, therefore resulting in fairly slow operation. The maximum speed touted for a Cassoli-type system is 12 packages per minute, although slower speeds are normally attained.
The system of the present invention overcomes all problems of the prior art by having two or more independently driven die carrying systems (axes) with each axis having one or more die sets. This provides significant advantages:
1. Extremely long dwell times are permitted, allowing the appropriate heating and cooling required for high speeds. PA1 2. The incoming product and film are driven at a constant velocity, with the individual sealing dies slowing to the point of closure on the film in order to accommodate the change in pitch between the products being sealed. All dies downstream of the closing die follow the speed of the closing die. Thus, there is constant velocity of the film and product to the first sealing die, and then variable velocity from then on during the sealing process. The upstream film is not stretched or jerked by such a process. PA1 3. Speeds of 45 packages per minute are attainable in a system according to the invention having three die sets in contact with the film at any one time, while providing 4 seconds of sealing and cooling time, more than adequate to obtain a seal of proper strength and integrity.
Because the dies of the invention have a variable velocity which can be programmed, the multi-axis, multi-die design of the present application is not confined to traditional horizontal form, fill and seal machines. For example, the concepts of the present application can be employed in sealing machines using two rolls of film, one roll above and one roll below a conveying plane, where heat sealing jaws seal the films together to form a curtain of film. In such apparatus, the product or products to be packaged are pushed through the curtain of film and the sealing dies come together to seal and cut, forming another curtain and also completing the package at the same time. Very often such packages are wrapped in a shrink film and then sent through a shrink tunnel. The concepts of the present application can be employed to increase considerably the speed of this type of machine.